It’s easy to change your birthsign in Morrowind. To change your birthsign in Morrowind press ~ to get the console window up, then type enablestatreviewmenu and hit enter. Use the window that pops up to change your birthsign then click OK. Changing your birthsign will set you back to level 1 so type player->setlevel x, where x was your level before changing your birthsign. Next out have to save the game then load the save to finish the level change.

❗ Note: It seems that changing your level also removed all your spells, so you’ll have to re-add them using the console or in-game.